1. Field of the Invention
The present invention relates to an electron gun for a cathode ray tube, and more particularly to an improved cathode structure in the electron gun which is capable of optimizing a space or gap between a sleeve and a heater received in the sleeve in order to improve an overshoot in emission of electrons.
2. Description of the Prior Art
Generally, such an electron gun is disposed in a neck formed at the rear of a cathode ray tube and provided with a cathode for emitting electron beams, so as to scan electron beams on a phosphor screen formed at the front of the cathode ray tube. This cathode should have such a structure that defines properly a space or gap between a heater and a sleeve, in order to maintain an overshoot in emission of electrons within an allowable range.
In conventional cathode structures, however, the gap defined between the heater and the sleeve is small, thereby causing the occurrence of an overshoot in emission of electrons exceeding the allowable range. This disadvantage will now be described, in conjunction with one example of conventional cathode structure illustrated in FIG. 1.
As shown in FIG. 1, the cathode structure comprises a sleeve 3 having a cylindrical shape and a heater 5 disposed in the interior of sleeve 3. The heater 5 includes a heating coil wire wound in the form of coil spring with a proper outer diameter D1 and a proper height L. The sleeve 3 is provided at the upper portion thereof with a cap 2 which is coated at its outer top surface with a layer 1 of an electron-emitting material such as carbonated oxide. The sleeve 3 is also provided at the lower portion thereof with a holder 4 through which a pair of heater taps 5a and 5b extends upwardly to be connected to the heater 5. The heater 5 has an outer diameter that is about 79% to 85% of the inner diameter D2 of the sleeve 3.
With this construction, as drive voltage of about 6.3 V is applied to heater taps 5a and 5b, the heater 5 generates heat of up to about 800.degree. C. by the drive voltage and applies the generated heat to the sleeve 3 and the cap 2. The cap 2 then transfers the heat from the heater 5 to the electron-emitting material layer 1. By the heat transmitted from the heater 5 via the cap 2, the electron-emitting material layer 1 emits thermal electrons. In the initial operation state, the electron-emitting material layer 1 emits an amount of electrons that is about 140% to 127% of the amount of electrons in the normal operation state. And a long time is taken until the amount of emitted electrons is maximized after the application of drive voltage to the heater taps 5a and 5b. This is because the amount of heat transferred from the heater 5 to the cap 4 includes the amount of heat by heat conduction. In this case, the generation of heat by heat conduction is caused by a reduction in the gap defined between the inner surface of the sleeve 3 and the outer surface of the heater 5. The reason for this reduced gap is that the heater 5 and the sleeve 3 expand abruptly, due to sudden heating of the heater 5. The gap has a size in the normal state when the heater 5 generates heat of a normal temperature.
As a result, the conventional cathode structure has a disadvantage that the size of gap between the heater 5 and the sleeve 3 is excessively reduced due to expansion of the sleeve 3 and the heater 5, in the initial operation state, that is, when the heater 5 suddenly generates heat. Due to this excessive reduction in the size of gap, the conventional cathode structure has a problem in that the amount of electrons emitted from the electron-emitting material layer 1 in the initial operation state cannot be maintained within an allowable range, that is, about 100% to 120% of the amount of electrons emitted in the normal state.